There has been a change in the update URL, checked in for trunk thus far only, adding a system-capabilities component (see bug 1274721 for details). Thus, nightly updates may or may not work tomorrow (especially if your CPU doesn't have SSE2 support).

Not likely, except for some fringe cases (nothing coming into my mind). On Linux, having 64-bit builds makes sense given that not everybody installs the 32-bit runtime libraries. On Windows, there is a dedicated WOW64 layer for 32-bit applications, which may introduce for those compared to native 64-bit builds, but I don't know if it has a quantifiable impact.